What does a "materials exposure experiment" do? The obvious answer is that it exposes materials to space. But the OPM did much more than that. It had the capability to take measurements on the materials being exposed while they were still out in the space vacuum, without having to bring them back to Earth and risk their being contaminated by handling and exposure to atmosphere and gravity.

The OPM monitored the space environment in its
vicinity, so it could ascertain what the samples were
actually being exposed to. And it had the
ability to be controlled from Earth, so
that the experiment could be changed on the
fly when unusual results were seen. From April to December 1997, the OPM, in its position on the outside of the Mir's Space Shuttle docking module, monitored the space environment around Mir and took measurements on material samples.